Prestressing apparatus

ABSTRACT

A method of and apparatus for banding a wall of a structure with a tensioned tendon. The apparatus comprises a carriage movable about the wall of the structure and a tendon tensioning means for stressing the tendon while the tendon is held and moved along a substantially straight line path of movement aligned with a tangent to a point at which the tendon is banded on the structural wall.

United States Patent Magers et al.

[ 1 Aug. 29, 1972 [54] PRESTRESSING APPARATUS [72] Inventors: LeroyMagers, Setauket, N.Y.; John H. Riddle, Del Mar, Calif.; August 1).Eitzen, Ville Centre, N.Y.

[73] Assignee: Gulf General Atomic Incorporated,

San Diego, Calif.

22 Filed: June 25,1970

21 App1.No.: 49,628

52 U.S.Cl. ..242/7.21, 226/172, 226/195, 242/147R 51 1m. (:1...B21f17/00,B21f45/00 Field of Search ..242/7 .21, 7.22, 7.23, 7.02,242/156,156.2,195,172, 155 BN, 147 R, 75.53

[56] References Cited UNITED STATES PATENTS 2,164,596 7/1939 Simonds..242/75.53

2,372,060 3/1945 Crom ..242/7.21 X 2,425 ,496 8/ 1947 Tyler ..242/75 .532,785,866 3/1957 Vogt ..242/7.21 3,010,631 11/1961 Gretter ..226/1723,221,401 12/1965 Scott et a1 ..242/7.23 X 3,227,344 l/ 1966 Rutter..226/195 X Primary Examiner-Stanley N. Gilreath AssistantExaminerMilton Gerstein Attorney-Anderson, Luedeka, Fitch, Even andTabin [57] ABSTRACT 27 Claims, 13 Drawing Figures e0 92 5:3 l-///l/Patented Aug. 29, 1972 9 Sheets-Sheet 1 S 8 v medfim v 0Q I] W m 00 5 9M y w M .5. 3 w 5 Am. mm mm B. 00 ri N0. H 0.. nc. 7 1 .PQ P P- 1a m2 8.0W0 no 3. 0m I a rm Na 0 0N0 18 mm; 0 v mm F s g Patented Aug. 29, 1972'9 sheets-sheets v llll rh Patented Aug. 29, 1-972 9 Sheets-Sheet 5FIG?- l III I I I I I I Ir Patented Aug. 29, 1972 9 Sheets-Sheet 6 QQEPRESTRESSING APPARATUS This invention relates to an apparatus and methodfor prestressing by banding continuous structures circumferentially withtensioned tendons and more particularly to an apparatus and method inwhich a carriage moves relative to a wall of the structure whiletensioning the tendon and paying out the tensioned tendon to thestructural wall.

The art of prestressing is well known, well established and has provenin many applications to be successful. In one example of prestressing,high tensile strength tendons are wrapped about a continuous wall of astructure of any suitable material, such as one made from reinforced orun-reinforced concrete or steel, with the tendons stressed in tension sothat the wall of the structure is placed in compression before anysubstantial load is developed against the wall. When an internal load isplaced against the wall, the compressive stress developed in thestructure must first be overcome before the wall goes into tension andadded tensile stress or tension is developed in the tendon.

Many methods have been suggested heretofore for continuouslyprestressing structures. In addition, various kinds of apparatus havebeen proposed to carry out the methods. One system involves thedevelopment of a tensile load in the tendon by means of passing itthrough a die to actually deform the tendon. Such a system is disclosedin US. Pat. No. Re. 22,762, issued June 4, 1946. Another system requiresthe tendon to be wrapped around a wheel or a capstan where a series ofindependently operated clamps grasp the tendon and develop a stress init as it travels about the wheel or capstan. Such a system is disclosedin US. Pat. No. 3,229,924 to Vogt, issued Jan. 18, 1966. Another capstanor wheel system in which several convolutions of a tendon are wrappedaround a wheel is found in US. Pat. No. 2,711,291 Kennedy, issued June21, 1955. The wheel or capstan apparatus and methods, like the diesystem, cause a non-even stress development in the tendon since theouter portions of the tendon are bent and curved about the wheel orcapstan and, hence are stressed to a greater degree along the outer sideof the tendon than along the inner side of the tendon. This stressingduring bending results in non-even distribution of stresses across thetendon.

Also, many of these prior art methods and kinds of apparatus are oflimited usefulness for large size tendons and for banding largestructures for which the operational speed of the apparatus becomesimportant. Thus, for large structures large quantities of tendon must betensioned and payed out at fast operational speeds for a commerciallyfeasible method.

Accordingly, a general object of the invention is to provide an improvedapparatus and method for prestressing a structure.

Another and more specific object of the invention is to tension a tendonmore uniformly across its cross sectional area by stressing the tendonwhile the tendon is moving along a path which is aligned substantiallywith a line tangent to the wall of the structure.

A further object of the invention is to provide an articulatedtensioning mechanism on a carriage movable about the structural wall inorder that the tendon may be stressed and maintained on the tangentialpath for different diameter structures or during a build up ofsuccessive convolutions of tendons on the structural wall.

Other objects and advantages of the invention will become apparent fromthe following detailed description taken in connection with theaccompanying drawings in which:

FIG. 1 is an illustrative view of a structure and an apparatus forprestressing the same and capable of carrying out the method of theinvention;

FIG. 2 is a partially schematic perspective view of an apparatus inaccordance with the present invention shown in relation to a continuouswall of a structure;

FIG. 3 is a top plan 'view of a tensioning mechanism and support of thepresent invention;

FIG. 4 is a partially sectional side view of the tensioning mechanism ofFIG. 3;

FIG. 5 is a schematic side view of the tensioning mechanism and verticalalignment system of the present invention;

FIG. 6 is an enlarged sectional view through the tendon and tensioningmechanism;

FIG. 7 is an enlarged sectional view showing the linkage arrangement forthe restraining elements of the prestressing mechanism;

FIG. 8 is an enlarged sectional view of a means for accommodatingelongation of the tendon during stressing;

FIG. 9 is a fragmentary top view of the restraining elements;

FIG. 10 is a fragmentary side view showing the interrelationship of therestraining elements;

FIG. 11 is a schematic flow diagram showing the drive arrangement forthe tensioning head and track assemblies;

' FIG. 12 is a view similar to FIG. 6 but showing a flat strip tendonbeing prestressed; and

FIG. 13 is a view similar to FIG. 8 but for prestressing a flat striptendon.

As shown in the drawings for purposes of illustration, the invention isembodied in an apparatus 11 for banding a wall 13 (FIG. 2) of astructure 15, such as a restraining barrier or tank, with stressedtendons 17 which are used to develop compressive stresses in the walland to prestress it circumferentially before an internal pressure isapplied within the structure. The apparatus is particularly suited forbanding very large structures, such as for example 40 ft. to 200 ft.diameter'cylindrical vessels at relatively high speeds with large sizetendons-although the apparatus is useable with smaller or larger sizesof both tendons and structures.

Generally, the apparatus 11 includes a carriage 19 having a tendontensioning means or mechanism 21 thereon for banding the wall 13 by themethod steps of: continuously moving the carriage 19 along a pathadjacent the wall 13 of the structure 15, paying out the tendon 17 fromthe carriage along a straight line which is tangential to the structuralwall, and tensioning and prestressing the tendon 17 while it is movingalong a path substantially aligned with this straight line tangential tothe wall. With the method and apparatus of this invention, the tendon isstressed without being bent or distorted while in-line with a tangentline from the tendon tensioning means to the point at which the tendonis banding on the wall or other previously laid tendons in one of aseries of grooves 13a 13d. Thus, with the present invention, the tendonreceives a more uniform stress distribution than is achieved withconventional tendon stressing operations such as with a die or with arelatively small radius wheel or capstan about which the tendon is bentwhile being stressed. Such bending while stressing causes a non-evenstress distribution across the diameter or equivalent dimension of thebent tendon.

Also, in accordance with the present invention, the tendon tensioningmechanism automatically adjusts its position to remain aligned with thestraight line tangential path to accommodate increasing diameters causedby overlapping a series of previously laid tendon convolutions; and alsoto permit its use with structures having different diameters. To theseends, the tendon tensioning mechanism 21 may be pivotally mounted on thecarriage 19 to swing automatically into alignment with the tangentialpath under forces exerted by the tensioned tendon itself. Also, thepivoting of tendon tensioning mechanism 21 on the carriage 19advantageously assists in isolating the tendon stressing mechanism fromvariations in movement of the carriage toward or from the wall 13 as thecarriage 19 travels about the wall.

Also, as will be explained in detail, the preferred tendon tensioningmechanism 21 may be shifted vertically on the carriage 19 so thatsucceeding vertically spaced wraps may be laid adjacent one another in agroove of the structural wall or in one or more of adjacent grooves 13a13d without shifting the carriage vertically. Means are also provided onthe carriage 19 to abut the wall and hold the carriage against rotation,that is twisting or turning when the tendon tensioning mechanism 21 isshifted vertically from a centered position on the carriage 19. Thecarriage 19 is driven along the wall by a traction drive means 26 whichpreferably includes caterpillar tracks 28 which have wide surfacecontact with the outer face wall 13 to distribute and reduce the loadper unit of surface area and which provide good tractive force toprestress relatively large tendons while propelling the carriage atoperational speeds. The traction drive means 26 also may be driven inthe reverse direction to remove thetendon 17 from the wall 15. That is,the tendon tensioning mechanism is driven in a reverse manner and thecarriage is moved in the reverse direction to remove tendon from thestructure while maintaining tension in that portion of the tendon notyet removed. On the other hand, in conventional kinds of apparatus thetendon becomes slack when the tendon is taken up.

Other and important features of the invention are a selective control ofthe prestressing of the tendon 17 by the tendon tensioning mechanism 21in which tendon engaging holding or restraining elements 27 (FIG. 6) arecarried on endless bands or carriers 29 to move into engagement with thetendon 17, travel therewith while restraining and prestressing thetendon, and then release the tendon to return to engage another upstreamportion of the tendon. The tendon tensioning means accommodateselongation of the tendon during stressing to prevent slippage of therestraining elements along the surface of the tendon, i.e., therestraining elements 27 are mounted in a manner that the restrainingelements 27 may move relative to the endless bands during elongation ofthe tendon. Also, the tendon engaging surfaces 30 on restrainingelements 27 of the restraining mechanism may be of a softer materialthan the tendon material to prevent damage to the surface of the tendon.Additionally, the restraining force applied by the tendon tensioningmechanism 21 may be selectively automatically controlled to obtain thedesired prestressing.

Referring now more specifically to the details of the illustratedapparatus 11, the carriage 19 (FIG. 2) is preferably formed with twounits or sections, namely a supply or service section or unit 33 and atrailing prestressing unit or section 35. The position of the units maybe reversed, if desired so that the supply unit follows the prestressingunit. The units 33 and 35 are connected in tandem for simultaneoustraversal along the wall 13, preferably by means of upper and lowerconnectors in the form of bars 37 which are fixedly secured at one endto the prestressing unit 35 and pivotally connected to the supply unitby pivot pins 39. The pivot pins 39 are disposed vertically in alignmentto define an axis allowing the units to pivot independently in ahorizontal plane and relative to each other. The tendon prestressingmechanism 21 is carried by the prestressing unit of the carriage and asupply reel 41 carrying a coil of unstressed tendon 17 is carried on thesupply unit. Thus, irregularities in movement of the supply unit 33 inthe horizontal plane are not directly transmitted to prestressing unitas if the units were rigidly attached to each other.

The carriage 19 is held inwardly and against the structure wall 13 bymeans in the form of one or more restraining bands 43 which are endlessloops passing around the structure and over and through the carriage 19.Preferably, the restraining bands 43 are held against the wall bytension and exert radial inwardly directed forces on the carriageholding the carriage against the wall 13. The upper restraining band 43extends across the upper and outer side of the carriage and is guided byand disposed within upper pairs of grooved sheaves 45 rotatably mountedon the respective prestressing and supply units. In a similar manner,the lower restraining band 43 extends across and is guided within a pairof sheaves 47 similarly mounted on both the prestressing and serviceunit. The restraining bands may be suitably tensioned by means such as atake up mechanism (not shown) on the sheaves.

The carriage 19 is propelled about the wall 13 by a traction drivemechanism which includes upper and lower caterpillar track assemblies 26each having an endless caterpillar track or tread 28 extending between apair of rollers 53 and 54 which are rotatably mounted on a stationarysupport frame 55 of the prestressing unit 35 for frictional engagementwith the wall 13. The caterpillar tracks 28 provide excellent tractionand permit distribution of the load over a relatively wide surface ofthe wall which is of particular advantage to reduce the load per unitarea on the wall 13 when stressing large tendons with relatively hightension forces. The caterpillar treads are driven by suitable motorssuch as, for example, hydraulic motors 56 attached to and in drivingengagement with rollers 53 of track assemblies 26. As will be explainedin greater detail hereinafter a regenerative hydraulic system is used todrive the hydraulic motors 56 and the caterpillar tracks 28 although itis to be understood that a mechanical or electrical motor drive systemmay be substituted for the hydraulic system. In addition to thecaterpillar tracks, the carriage is provided with several rollerassemblies 59 and wheel assemblies 61 for rolling about the surface ofthe wall 13 while the carriage 19 is being driven by the respectivecaterpillar assemblies 28.

To resist moments tending to twist the prestressing unit 35 and to guidethe carriage in a predetermined path, in a horizontal plane in thisinstance, the carriage is provided with several wheels and rollers whichride on the outside surface of the wall 13 and rotate aboutsubstantially vertical axes and others which project into the grooves13a 13d and roll along a top wall 62a or bottom wall 626 of a groove. Inthis illustrated embodiment of the invention, the leading portion of theprestressing unit has an upper and lower roller assembly 59 each havinga vertically disposed wheel or roller 63 projecting radially inwardlyinto a groove in the wall 13 to roll on a top or bottom groove wall 62aand 62b while turning about a horizontal axis. More specifically, theroller 63 of the upper wheel assembly 59 engages a lower groove wall 62band the roller 63 of the lower wheel assembly engages an upper groovewall 62a and the rollers 63 roll along the outer edge of theirrespective grooves while turns of tendon are laid within another groovebetween these grooves. The spacing of the rollers 63 and theirengagement with the horizontally disposed groove walls 62a and 62bresists twisting of the prestressing unit 35. The roller 63 is mountedon a bracket 69 secured to the prestressing unit 35 and a pair of spacedrollers 71 and 73 are carried by the same bracket for turning aboutsubstantially vertical axes while rolling along the outer face wall 13of the structure between a pair of grooves. The service unit 33 isprovided with four wheel assemblies 61 which have spaced wheels 74 and75 for riding on the outer face wall 13 between grooves. The respectiveroller and wheel assemblies 59 and 61 are preferably mounted onadjustable mechanisms (not shown) for moving them radially inwardly oroutwardly and for moving them vertically to assure that their respectivewheels and rollers are properly positioned for good rolling contact withthe structure which may vary in configuration or dimensions between thetop or bottom thereof. Also, this allows the same carriage to be usedwith different kinds and sizes of structures.

After banding several of the grooves 13a, 13b etc., it is necessary todisplace the carriage along the wall 13in a vertical direction toposition the same for banding a next series of grooves in the wall 13.The preferred manner of supporting and positioning the carriage 19vertically is by means of a suspension system 77 (FIG. 2) which issecured on the structure in this instance. It is to be understood thatthe carriage 19 may be supported and positioned for verticaldisplacement on a frame or suspension system which is independent of thestructure 15. The illustrated suspension system 77 comprises an overheadtrack 78 in the form of a circular I beam secured to and supported atthe top of the wall 13 by radially and inwardly directed beams 76.Mounted for rolling on and along the overhead track 78 are a series oftrolley units 79 having trolley wheels 80 guided for movement within anoutwardly facing groove 81 in the track 78. In this instance, front andrear trolley units. support the forward and rearward ends of therespective service and prestressing units 33 and 35. The wheels willroll on a lower wall in the groove 81 of the track while the carriage istraversing about the wall 13. Depending from the trolley units 79 arefree-turning pulleys 82 about which extend support cables 83 having oneend 84 anchored to the frame of the service unit 33 or the frame of theprestressing unit 35. The other end of each of the respective supportcables is wound about and fastened to power drive hoists 85 which may bedriven to wind or unwind the cable thereon thereby adjusting the lengthof cable extending from the hoist 85 to the pulley 82 and down to theanchored end 84. In this manner, the carriage 19 is positionedvertically and the trolley units 79 support the weight of the carriagewhile rolling in the track 78 while the carriage is propelledcircumferentially about the wall 13 by the traction caterpillar tracks28.

During such movement of the carriage 19 about the wall 13, the tendon 17is continuously prestressed and payed out from the tendon supply reel 41on a stand 93 carried on a floor 95 of the service unit 33. Theillustrated tendon 17, as best seen in FIG. 6, is of the multistranded,circular cross section kind although the apparatus 11 is capable of usewith tendons of various other sizes and cross sectional shapes such as,for example, fiat rectangular cross section as shown for a flat striptendon 17a, as shown in FIGS. 12 and 13. While the size of the tendonmay vary considerably, the apparatus l 1 is particularly useful forrelatively large sizes of tendons such as the illustrated seven strandtendon 17 which, in this instance, may have a diameter of 0.6 inch. Theillustrated flat strip tendon 17a may typically have a 6 inch width anda one thirty-second inch thickness. The length of tendons 17 or 17astored on the supply reel 41 is quite long in order that large diameterstructures, e.g., 40 to 200 ft. in diameter may be banded quicklywithout frequent stops to change supply reels. Also, long lengths oftendon on a supply reel allow the carriage 19 to maintain relativelyhigh speeds, e.g., the minimum operating speed for the carriage in theillustrated embodiment of the invention may be as much as ft. per minutefor 20 ft. dia. structures and 300 ft. per minute for 80 ft. dia.structures. From the reel 41, the tendon 17 is fed through the tendontensioning mechanism 21 which will now be described in detail.

Referring now to the tendon tensioning mechanism 21 and moreparticularly to its articulation on the tendon prestressing unit 35,this articulation assures an in-line path for the tendon 17 while beingstressed, i.e., in-line with a tangent to the point at which the tendonis being banded to and about the wall 13. The tendon prestressing unitcomprises a box like frame 55 (FIG. 2) having upper horizontallydisposed plates 100 and 101 and a lower horizontally disposed floorplate 102 and bars 103 which are joined by upright rigid bars 105. Thetendon tensioning mechanism 21 is carried on a stand 107 fastened at itslower end to a floor plate 102 of the prestressing unit.

The tendon tensioning mechanism 21 is pivotally mounted on a carrier 108carried on the stand 107 for pivoting about a vertical axis through apair of vertical stub shafts 109 and 110, as best seen in FIGS. 2, 3 and4 on the carrier. The stub shafts have outer ends journaled in bearings111 fastened to top and bottom carrier plates 113 and 114. The stubshafts 109 and 110 have inwardly projecting ends fastened to the top andbottom of a bearing block 1 (FIG. 4) which supports in cantileverfashion a supporting frame 117 for the endless chains 29 and therestraining elements 27. More specifically, the frame 117 iscantilevered by means of a pair of vertically spaced upper and lowersupport bars 121 and 123 which extend between the bearing block 115 andthe frame 117 with theupper support bar 121 bolted to a top frame plate125 and the lower support bar bolted to a lower frame plate 127 of theframe 117. As will be explained in greater detail, the tendon 17 actingthrough the restraining elements 27 and chains 29 exerts turning momentson its supporting frame 117 which in turn acts through the support barsto exert turning moments on the bearing block and attached stub shaftsfor turning the latter in the bearings 111 on the carrier 108.

For the purpose of measuring and controlling the force of restraintapplied by the prestressing mechanism, the support bars 121 and 123 arepreferably slidably mounted for longitudinal movement in slide bearings126 and 128 carried in the bearing block 115. More particularly, loadmeasuring cells 137, as best seen in FIG. 3, are acted upon by andmeasure the force of restraint tending to pull and slide the supportbars 121 and 123 to the right as viewed in FIGS. 2, 3 and 4. The loadcells 137 are disposed between the non-slideable, bearing block 115 anda vertical, rear plate 129 which'is attached to the right hand ends ofthe support bars 121 and 123 to move therewith. As

best seen in FIG. 3, the bearing block 115 has a pair of horizontallyextending wings 141 to which are fastened the load cells 137 withplungers 143 of the load cells extending to and abutting opposing facingsurfaces'145 on the vertical plate 129. Thus, the restraint applied tothe tendon 17 will cause the vertical plate 129 to apply forces to theplungers 143 of the load cells 137 and hence measure the restraintapplied thereat.

The bars 121 and 123 support the tendon restraining mechanism 21 in acantilever manner; and the latter may be provided with a counterweightsystem 147, as best seen in FIG. 5, for providing greater stability andfor applying a lifting force to the outer unsupported (right) end of thetendon tensioning mechanism 21. Fixed to the frame 117 of the tendontensioning mechanism in a position towards the unsupported end thereofis an upstanding bracket 149 to which is connected a chain 151 which ispassed around a pair of freely rotatable sprockets 153 mounted on theframe member 101 of the prestressing unit. The chain is then passedthrough an opening in the carrier 108 and about another sprocket 157rotatably mounted on the floor plate 102. The chain extends upwardlyfrom the sprocket 147 and is secured at 156 to the under portion of thecarrier. Since chain 151 is of a fixed length, it will maintain thesupporting frame 117 of the tendon tensioning mechanism in substantiallythe same attitude with respect to the structure 15 when the carrier isshifted vertically, as will be explained. The sprockets 153 and 157 maybe suitably mounted for swiveling to prevent interference with pivotalmovement of the tendon tensioning mechanism 21.

In many instances, it is desired to lay tendons 17 closely adjacent toone another in the grooves 13a, 13b, 13c and 13d in the structural wall;and this may be accomplished without vertically displacing the carriage19 as the carrier 108 for the tendon tensioning mechanism 21 is mountedfor sliding vertically on the stand 107 of the carriage. Also, severalsuccessive convolutions of tendons may be wrapped. in the same groove byshifting the carrier 108 without displacing the entire carriage l9vertically along the wall 13'. The vertical stand 107 is comprised of apair of upstanding standards 158 each having an outer tubular channel159 (FIG. 3). The standards 158 are bolted at the upper and lower endsthereof to the top and bottom frame plates 101 and 102 as best seen inFIG. 4. As best seen in FIG. 3, the carrier108 has four slide pads 162-engaging each of the four sides of the standards 158 for slidingtherealong and guiding the carrier for straight line vertical movement.

The prestressing carrier 108 is shifted vertically on the carriage 19 bymeans in the form of a screw or jack drive mechanism including a pair ofvertically extending screw jacks 165 (FIGS. 3 and 4) spanning the topplate 101 and bottom plate 102 and journaled for turning in bearings 167secured to the respective top and bottom plates. The screw jacks arethreaded in a pair of nuts in the form of threaded blocks 169 fixed tothe carrier 108 and the threaded blocks travel vertically on the screwjacks as they are turned and thereby the carrier 108 and prestressingmechanism 21 are shifted vertically along the standards 158.

To turn the screw jacks 165, a screw drive 171 (FIG. 4) is provided andit comprises a gear box 172 driven by a belt 173 which is connected to atake-off hub 175 of a variable speed reducer 177 which is driven by amotor 179 through a belt 180. The illustrated motor 179 is an electricone although it may be of the hydraulic type if desired.

As the prestressing carriage 19 travels about the wall 13, the screwjacks 165 may be rotated so that the carrier 108 is gradually liftedupwardly (when the tendon is being wrapped from the bottom upwardly.) Inorder to insure that the rate of rise of the carriage 19 is controlled,means to sense the position of the carriage are provided in the form ofa feeler arm 181 (FIG. 3) which will ride on the bottom horizontalsurface of a groove until the first convolution of the tendon iscompleted and wrapped about the wall. Thereafter the feeler arm 181rides on the last convolution of the tendon which has been laid. Thefeeler arm 181 is an articulated arm which is attached to the free endof the stressing mechanism 21. Also the feeler arm 181 is operativelyconnected to a suitable motor control device such as a micro switch 183(FIG. 4) for an electric motor 185 which is driven in a manner to adjustthe ratio of the variable speed reducer 177 to cause the rate ofvertical climb of the carrier 108 to be increased or decreased asnecessary.

After leaving the supply reel 41, the tendon 17 is guided through thecarrier 108 to the tendon tensioning mechanism 21 by a series of guiderollers including a first pair of vertically oriented, groove rollers186 (FIGS. 3 and 4) rotatably mounted for turning on a bracket 187attached to the carrier and a second set of horizontally orientedrollers 188 mounted for turning about spaced vertical axes on thebracket 187 which is fixed to the carrier. The first set of rollers 188are mounted for rotation about horizontal axes and the second set ofgrooved rollers 186 are mounted for rotation about vertical axes with apathway being defined between the grooved rollers 186 beingsubstantially in alignment with an opening 189 in the bearing block 115.After passing through the bearing block; the tendon travels betweenanother set of horizontally oriented, grooved rollers 190 which guidethe incoming tendon to its proper position between the chains 29 andrestraining holders 27.

In the tensioning mechanism 21, the tendon 17 is stressed whilecontinuously moving therethrough by means of the traveling restrainingelements 27 defining a pathway for the tendon. The amount of restraintapplied by the holding elements 27 to the tendon 17 determines thedegree to which the tendon is prestressed and also its degree ofelongation.

While the tendon 17 is passing through the tendon tensioning mechanism21, a series of the restraining elements 27 will simultaneously beengaging the tendon, traveling with the tendon, and exerting arestraining force on the tendon. Each of the respective tendonrestraining elements has a vertically disposed block 191 which spans apair of vertically spaced upper and lower, endless roller chains 29, asbest 'seen in FIGS. 6 and 7. The chains carry the blocks 191 about anendless path including an inner run at which the tendon 17 is engagedadjacent an inlet sprocket 195 (FIG. 3) to outlet sprocket 197 at whichthe restraining elements release the tendon and return along the outsidepaths or runs to the inlet sprockets 195. Each of the inlet sprockets195 is a double sprocket and each of the chains 29 is a composite chainhaving an upper roller chain 196 and a lower roller chain 198. Eachinlet sprocket 195 is fixed to a vertically extending shaft 199 which isjournaled in bearings fixed to top and bottom frame plates 125 and 127of the tensioning mechanism. In a similar manner, each of the outletsprockets 197 is a double sprocket and each is attached to a common,vertically extending shaft 201 which is journaled in bearings 203carried by stationary frame plates 205 and 207 (FIG. 4) of thetensioning mechanism. The rotational axes of the inlet and outletsupport shafts and sprockets are positioned on opposite sides of thetendon 17 so that the restraining elements 27 move along a substantiallystraight line, parallel paths between the inlet and outlet sprocketswhile engaging the tendon 17.

The restraining blocks 191 are attached to the upper and lower chains196 and 198 by elongated pins 207 (FIG. 7) which extend from the upperchain through suitable openings in the blocks to the lower chain. Theselong pins which extend between the chains a1- temate with short chainpins 209 which serve with the long pins 207 to fasten the respectivechain links 210 in a given chain together. Suitable chain rollers 211are mounted for rotation on each of the respective long pins 207 andshort pins 209 between chain links 210 in a familiar and conventionalmanner.

The restraining elements 27 are mounted on the composite chains 29 in amanner to permit the tendon surfaces 30 thereon to move relative to thechains to accommodate elongation of the tendon 17 as it is being takenfrom substantially zero tension to full tension but without the tendonengaging surfaces 30 slipping on the tendon during such elongation. Tothis end, the restraining elements 27 are mounted in a manner to shiftrelative to the chains and preferably to shift against the bias of aspring means 213 which permits rearward shifting of restraining elements27 relative to the chains upon compressing the spring means 213. In thisinstance, the spring means is in the form of a series of curved, springdiscs 215, as best seen in FIG. 8, carried in a pair of bores 217 in acentral part of the blocks 191 of the restraining elements 27. Thespring discs each have a circular recess through which extends a guiderod 221 on the forward end of which is a plug 223 bearing against a longchain pin 207. The other end of the guide rod 221 is inserted in asmaller diameter bore 225 in a shoulder wall 227 against which bears therearward one of the spring discs 215. To assist the spring discs 215 insliding during compression or expansion thereof, a sleeve liner may beemployed with the spring discs 215 inserted therein and with the sleeveliner disposed within the bore 217. Preferably, a pair of such springmeans 213 are provided above and below the tendon path of travel as bestseen in FIG. 6.

As an alternative to the preferred embodiment of the invention, themovement of the restraining elements 27 relative to the supportingchains 29 may be limited during stress and elongation of the tendon 17to less than that needed to accommodate the elongation of the tendon ata given tension whereby the additional displacement needed toaccommodate full elongation of tendon is provided by slippage or partialyielding. For example, the spring discs 215 may be selected to bottomout at a tension load which is less than that of the given full tensionload; and the tension elements may have partially yieldable surfaces 30which will yield to provide an additional displacement needed toaccommodate the total elongation of the tendon at the given tension. Onthe other hand, the tendon may simply slip relative to the restrainingelements once the springs 215 have bottomed out and the slippage isallowed to accommodate the remainder of the elongation of the tendon.With these embodiments of the invention, however, there will be anaccompanying increase in the wear rate of the tendon engaging surfaces30.

Preferably, each restraining element 27 is interconnected with itspreceding and succeeding restraining element. More specifically, therear portion of each block 191 of the restraining element includes acentrally disposed tongue 231 (FIGS. 7 and 8) which projects rearwardlyinto a slot 233 forming a yoke on the front of the following block 191.Each tongue has an elongated opening 235 therein through which the longpin 207 extends to permit relative movement of the block relative to thepin. Elongated openings 237 are also formed in the yokes to receive thesame elongated pin 207 to permit recovery movement, i.e., forwardmovement of the block when the tendon is released and the spring discsare free to expand. Thus, each block 191 is connected to its precedingand succeeding block 191 and is free to move relative thereto.

To avoid damage to the tendon 17, it is preferred that the tendonengaging surface 30 be less hard than that material of the tendonengaged thereby so that the surfaces 30 will not scratch or cut thetendon surface.

should be no substantial change in major properties of the tendon suchas for example, a change in the ultimate strength, yield point or otherphysical characteristics due to damage to the tendon surface by thetendon engaging surfaces 30. Scratching or cutting is undesirable sincesuch scratches or cuts have a deleterious effect on the mechanicalproperties of the tendon. In particular such scratches or cuts may serveas a substantial source of fatigue failure. Preferably, the tendonengaging surfaces 30 may be provided with a groove sized to receivetherein a portion of the arcuate periphery of the tendon.

The restraining elements 27 are urged into tight gripping relation withthe tendon 17 to prevent slipping therebetween during the restraint by apressure means 238 (FIG. 3) which comprises a series of hydraulic jacks239 which are secured at one end to a stationary frame member 241 andwhich apply force at their other ends to a movable back up plate 243. Asbest seen in FIG. 6, each back up plate 243 carries a series of rollers245 which bear against a flat rear wall 247 of the blocks 191 of therestraining elements. Other alternative pressure means such as screwsand/or springs or other devices may be used in lieu of the hydraulicjacks to apply pressure to the restraining elements and thereby to thetendon 17. Only one set of hydraulic jacks may be used, if another backup plate 243a is secured and held to the first back up plate by tie rods249, as shown in FIG. 3. The rollers 245 provide reduced frictionsurfaces by which sufiicient force may be imparted to the movablerestraining elements 27 to prevent the tendon sliding on the surfaces 30thereof. The preferred restraining elements are sufficiently long thatthe blocks 191 extend across at least three sets of rollers 245 with theresult that the load from the hydraulic jacks 239 is distributed in alongitudinal direction along three spaced areas on each block 191. Asbest seen in FIG. 6, two vertically spaced rollers 245 are journaled ona common pin 251 to provide two vertically spaced contact areas. Thus,six rollers 245 are normally in contact with the surface 247 of eachblock. As will be explained in detail, the hydraulic jacks may each besubjected to the same hydraulic fluid force to provide a uniformlydistributed load or a non-uniform distribution of load may be achieved.For example, the hydraulic jacksmay receive progressively greaterhydraulic pressures so that the tendon 17 receives a substantiallylinear build up in stressing from the inlet to the outlet of the tendontensioning mechanism 21.

In order to reduce the size of the motor used to drive the carriageabout the structural wall and to prestress the tendon, a regenerativesystem 260 (FIG. 11) may be used in which the work developed within thetendon tensioning mechanism 21 is used to assist in driving therespective track assemblies 28 to propel the carriage 19 about thestructural wall 13.

As explained above, the upper and lower track assemblies 26 each have aroller 53 driven by a main hydraulic motor 56 which is connected by ahydraulic line 261 to a pump 263 driven by a prime mover 265 such as adiesel motor. A common hydraulic line 264 extends between the hydrauliclines 261 from the pumps 263 to provide a common system for the trackdrive motors 56. To assist in driving the track assemblies, aregenerative hydraulic motor 267 is attached to each of the caterpillarrollers 54 to assist the main hydraulic motors 56 in moving the carriage19 about the structural wall 13. The regenerative hydraulic motors aredriven by regenerative hydraulic pumps 269 which are connected by adrive 270 to sprocket shafts 199 of the tensioning mechanism and aredriven thereby to pump hydraulic fluid through hydraulic lines 271connected to the regenerative hydraulic motors 267. A common hydraulicline 272 interconnects the hydraulic lines 271 to assure that theregenerative motors 267 are being driven equally by a common hydraulicsystem. Thus, the work being done by the tendon tensioning mechanism 21is used to drive the regenerative hydraulic pumps 269 and regenerativehydraulic motors 267 to assist in propelling the carriage 19 therebydecreasing power requirements necessary for the main pumps 263 forpropelling the carriage l9.

The manner of controlling the amount of tension in the tendon 17 mayalso be controlled with the regenerative system 260 as by eitherincreasing or decreasing the amount of torque allowed to be done by theregenerative hydraulic pumps 269 and motors 267. More specifically, theload cells 137 which measure the tension in the tendon 17 maybeconnected to a mixing amplifier 273 which in turn is connected to servovalves 275 which regulate the angle of swash plates within theregenerative pumps 269 thus determining the torque produced by the pumps269 for transmittal to the regenerative motors 267.

For the purpose of insuring that the regenerative hydraulic pumps 269are actually transmitting the desired torque to the regenerativehydraulic motors 267, feedback transducers 277 may monitor the pumps 269and provide outputs to a summing amplifier 279 which adds these signalstogether and compares their total to a reference signal for the desiredload. If further correction is necessary the summing amplifier 279causes the amplifier 273 to cause the servo valves 275 to be againchanged. Therefore, the actual tension being developed in the tendon 17is under constant and automatic surveillance and correction.

In the embodiments of the invention illustrated in FIGS. 12 and 13,tendon restraining elements 27a are provided and these are similar tothe above described tendon restraining elements 27 except for havingflat tendon engaging surfaces 30a for the flat strip tendon 17a ratherthan having the grooved tendon engaging surfaces-30 for the circularcross sectioned tendon 17. The remaining elements of the tendontensioning mechanism shown in FIGS. 12 and 13 are substantially similarto elements above described in connection with FIGS. l-ll and theidentical reference characters have been used for these similarelements.

In operation of the apparatus 11, the carriage 19 is propelled by thecaterpillar tractive drives 28 in a clockwise direction as viewed inFIGS. 1 and 2. With the right hand end of the tendon 17 anchored to thestructure, the tendon will extend in a straight line tangential path tothe wall 13 of the structure from the car riage. Within the travelingcarriage tendon is being uncoiled from the reel 41 and passed throughthe tendon tensioning mechanism 21 at which the tendon is being stressedin tension by opposed sets of traveling restraining elements 27 on thepair of endless bands or chains 29. The restraining elements are spacedapart by a distance determined by opposite sets of rollers 245 anddefine a line pathway for the tendon and this pathway is aligned withthe tangential line to the structural wall. Hydraulic cylinders 239 actthrough the rollers 245 to apply forces direct normal to the directionof tendon travel against the restraining elements 27 to prevent slippagebetween the latter and the surface of the tendon 17. Also, the multiplesets of restraining elements 27 provide multiple areas of holdingcontact to prevent slippage.

Usually, several convolutions of prestressed tendon 17 are laidvertically along each groove interior wall 620 and the innerconvolutions of tendon are overlaid with several outer convolutions oftendon until the desired amount of tendon is laid within a given groove.During the overlaying of convolutions within a groove, the tendontensioning mechanism 21 may pivot slightly in the clockwise direction,as viewed in FIG. 1 from a position, diagrammatically illustrated indotted lines in FIG. 1 to another position, diagrammatically illustratedin solid lines in FIG. 1 to accommodate a slight difference in angle dueto the difference in diameters of the respective convolutions of tendonwhich results in different angles for the tangent line path from thecarriage to the structure. If different parts of the structure havedifferent diameters, then the pivoting of the tendon tensioningmechanism 21 will be more pronounced for the different parts of thestructure.

Advantageously, all of the restraining forces applied by the tendontensioning mechanism 21 to the prestressing unit 35 are applied at thepivot axis of the tendon tension mechanism on the carrier 108 throughthe stub shafts 109 and 110. This tends to isolate the tendon tensioningmechanism 21 from radially directed movements of the prestressing unit35 and supply unit 33 and facilitates measurement of the prestressingforces by the load cells 137.

From the foregoing it will be seen that the apparatus 11 provides a morenearly uniform stress distribution and good control of stress applied tothe tendon. Also, the apparatus may operate at relatively fast operatingspeeds particularly for large sizes of tendons and for large sizes ofstructures. Additionally, a regenerative system provides economies inthe amount of power consumed for the prestressing operation. Also, theability to reverse the movement of the carriage while maintaining fulltension permits correction of an error in laying without unwrapping theentire length of cable.

While a preferred embodiment has been shown and described, it will beunderstood that there is no intent to limit the invention by suchdisclosure but, rather, it is intended to cover all modifications andalternate constructions falling within the spirit and scope of theinvention as defined in the appended claims.

What is claimed is:

1. Apparatus for prestressing a tendon and banding a circumferentialwall of a structure having a longitudinally extending axis with atensioned tendon, said apparatus comprising a carriage movable about thecircumferential wall of said structure, means for moving said carriageabout the circumferential wall, tendon tensioning means on said carriagehaving means engaging said tendon along an extended linear portionthereof for applying tensile forces thereto to stress the tendon, saidmeans engaging said tendon and applying tensileforces thereto along asubstantially straight line path of movement without substantial bendingstresses being applied to the tendon, and means pivotally mounting saidtendon tensioning means on'said carriage with said means engaging saidtendon along an extended portion thereof for turning about an axissubstantially parallel to the longitudinal axis of said structure toalign said tensioned-linear portion with a tangent line extending fromsaid tendon tensioning means to a point at which said tendon is handedon said structural wall.

2. An apparatus in accordance with claim 1 in which said tendontensioning means includes a series of traveling, opposed restrainingelements for engaging the tendon and for forming a pathway therebetweenfor said tendon, said last mentioned pathway being aligned with thetangent line to said structure wall.

3. An apparatus in accordance with claim 2 in which endless bands moveabout endless paths and in which said restraining elements are carriedby said endless bands to engage said tendon and move along said straightline path therewith while the tendon is being tensioned and in whichsaid restraining elements are carried into and from engagement with thetensioned tendon by said endless bands.

4. An apparatus in accordance with claim 1 in which said tendontensioning means includes means for engaging said tendon and fordisplacing with elongation of said tendon during stressing thereof toprevent slipping of said means along the surface of said tendon.

5. An apparatus in accordance with claim 1 in which means are providedon said carriage for displacing said tendon tensioning meanstransversely to the path of carriage movement to position adjacenttendons side by side while said carriage remains in a given path ofcarriage movement.

6. An apparatus in accordance with claim 5 in which means on saidcarriage engage said structural wall at spaced locations and hold saidcarriage against twisting on said structural wall after a transverseshifting of said tensioning means and tendon.

7. An apparatus in accordance with claim 1 in which said means formoving said carriage includes caterpillar tracks for engaging saidstructural wall and for providing a propelling force to move saidcarriage about said wall.

8. An apparatus movable about a wall of a structure for tensioning atendon and for applying the tensioned tendon to said wall, saidapparatus comprising a carriage movable about the wall of the structure,said carriage having a supply unit and a tendon tensioning unit,caterpillar track means on at least one of said units having tractiveengagement with the structural wall, means pivotally connecting saidunits for movement together as a tandem about the structural wall, asupply reel of tendon carried on said supply unit for providing tendonfor tensioning, and means on said tendon tensioning unit for tensioningthe tendon and paying out the tensioned tendon along a linesubstantially tangential to said structural wall.

9. An apparatus in accordance with claim 8 in which means on saidcarriage engage said wall at spaced points and hold said carriageagainst twisting on the structural wall.

10. An apparatus in accordance with claim 7 in which means are providedto shift said tendon tensioning means to permit a plurality of tendonsto be applied at longitudinally spaced positions along said structuralwall without shifting said carriage along said wall.

11. Apparatus for tensioning a tendon and for banding a wall of astructure with a tensioned tendon, said apparatus comprising a carriagemovable in a forward or reverse direction about the wall of thestructure, tendon tensioning means on said carriage for tensioning saidtendon while said tendon is moving along a substantially straightlinepath which is along a tangent to the structural wall, said tendontensioning means including a series of restraining elements for engagingand restraining said tendon and for traveling therewith while tensioningthe same along an extended linear portion of said tendon, means mountingsaid tendon tensioning means on said carriage for alignment with atangent line extending from said tendon tensioning means to saidstructure, said restraining elements being movable in one direction whensaid carriage is paying out said tendon and being movable in an oppositedirection when said carriage moves in the reverse direction to takeupsaid tendon and means for moving said carriage in the forwarddirection to pay out the tensioned tendon and in the reverse directionto take up the tensioned tendon while holding said tendon under acontrolled, predetermined tension.

12. Apparatus in accordance with claim 11 in which said means for movingsaid carriage includes a motor drive and caterpillar tracks for engagingthe structural wall and in which said tendon tensioning means includes apair of endless carriers for carrying said restraining elements intoengagement with said tendon to travel therewith and from said tendon forreturn to positions to re-engage said tendon.

13. A prestressing apparatus as defined in claim 12 wherein saidrestraining elements are slidably mounted and have tongue and yokeportions, said yoke portion including a pair of flanges, and the tongueportion of each element adapted to be slidably positioned within theflanges of an adjacent element.

14. A prestressing apparatus as defined in claim 12 wherein saidrestraining elements include a resilient member whereby the slidingmovement of the restraining elements in passing through the tensioningzone is substantially equal to the elongation of the tendon in passingthrough said zone.

15. A prestressing apparatusfor circumferentially tensioning a tendonabout a continuous structure by elongating the tendon a desired extent,said apparatus comprising a support carriage movable about saidstructure, a tendon tensioning mechanism carried by said carriage meansmounting said tendon tensioning mechanism on said carriage forarticulation about an axis to align said tendon tensioning mechanismwith a line tangent to said structure, and means for shifting themechanism along a path parallel to said axis to adjust said line oftangency relative to said carriage and to said structure.

16. A prestressing apparatus as defined in claim 15, wherein thetensioning mechanism includes a pair of opposed loops each comprised offlexible links engaging a pair of spaced, rotatable sprockets fixedlyposi tioned to maintain said loops taut, means for rotating a sprocketin each pair in timed relation whereby said loops define a pathway forthe tendon and create a zone for progressively increasing the tension insaid tendon as it passes through said zone.

7 17. A prestressing apparatus as defined in claim 16 wherein saidopposed loops include a plurality of ten- .don restraining elements forengaging the tendon throughout its travel along the pathway defined bythe opposed loops. j

18. A prestressing apparatus as defined in claim 17 wherein said tendonrestraining elements have a surface thereon for engaging the tendon, thehardness of said surface being less than that of the tendon.

19. A prestressing apparatus as defined in claim 16 and furtherincluding a back-up plate positioned within each loop and extendingsubstantially the length of the pathway formed by the loops, at leastone of said plates adapted to be moved whereby the pressure exerted bythe restraining elements on the loops against the tendon may be varied.

20. A prestressing apparatus as defined in claim 17 wherein saidrestraining elements are supported by a plurality of rollers whereby thedistance between the holding elements of the opposed loops is determinedby the distance between the opposed rollers.

21. A prestressing apparatus as defined in claim 15, in which a movablecarrier for the tendon tensioning mechanism'is provided on said carriageand said tendon tensioning mechanism is supported thereon in acantilevered condition, and in which a counterweight arrangementcomprising a flexible connector of a fixed length is attached to oneside of the cantilevered portion of the tensioning mechanism and passedover a roller fixed on the support carriage and attached to the carrieron the side thereof opposite the attached side of the cantileveredportion whereby said tensioning mechanism is supported by said connectoras the carrier is moved relative to the carriage.

22. A tendon mechanism for prestressing a tendon without substantiallybending the prestressed tendon before banding the tendon on a structurehaving a longitudinally extending axis comprising, a frame means, asupport means, a pair of opposed, endless bands mounted on said supportmeans for movement about endless paths, and a plurality of tendonrestraining elements mounted on said endless bands for travel therewithand for forming a straight line pathway for the tendon to travel along,said restraining elements engaging the tendon during its travel alongsaid straight line pathway, said tendon restraining elements exertingforces on said tendon to prestress the same while traveling along saidstraight line pathway and means pivotally mounting said support means onsaid frame means for pivoting thereon about an axis parallel to saidlongitudinal axis of the structure and relative to said structure beingbanded to align said tendon being tensioned with a tangent line to thestructure.

23. A tendon tensioning mechanism in accordance with claim 22 in whichsaid support means is slidably mounted on said frame means for movementin the direction of tendon travel and in which load sensing with claim24 in which means are provided to allow said restraining elements tomove relative to said endless bands during elongation of the tendon toprevent substantial surface slippage between the tendon and therestraining elements.

26. A tendon tensioning mechanism in accordance with claim 22 in whichmeans are provided to allow said restraining elements to move relativeto said endless bands for a predetermined displacement during tendonelongation and in which surfaces are provided on said restrainingelements for partially yielding to accommodate further elongation ofsaid tendon to its fully stressed length.

27. A method of tensioning a tendon and banding a circumferentiallyextending wall of a structure with the tensioned tendon by a tendontensioning mechanism carried on a movable carriage; said methodcomprising the steps of: moving the carriage circumferentially about thewall, tensioning an extended linear portion of said tendon while saidtendon is moving along a straight line path through said tendontensioning mechanism without producing a substantial bending of thetendon and without drawing the tendon to produce the tension therein,aligning the-tensioned linear portion of the tendon with a straight linetangent path from the tendon tensioning mechanism to the circumferentialwall of the structure, paying out said tensioned tendon along saidstraight line tangential path to said structure, and banding saidcircumferential wall with said tensioned tendon.

1. Apparatus for prestressing a tendon and banding a circumferentialwall of a structure having a longitudinally extending axis with atensioned tendon, said apparatus comprising a carriage movable about thecircumferential wall of said structure, means for moving said carriageabout the circumferential wall, tendon tensioning means on said carriagehaving means engaging said tendon along an extended linear portionthereof for applying tensile forces thereto to stress the tendon, saidmeans engaging said tendon and applying tensile forces thereto along asubstantially straight line path of movement without substantial bendingstresses being applied to the tendon, and means pivotally mounting saidtendon tensioning means on said carriage with said means engaging saidtendon along an extended portion thereof for turning about an axissubstantially parallel to the longitudinal axis of said structure toalign said tensioned linear portion with a tangent line extending fromsaid tendon tensioning means to a point at which said tendon is bandedon said structural wall.
 2. An apparatus in accordance with claim 1 inwhich said tendon tensioning means includes a series of traveling,opposed restraining elements for engaging the tendon and for forming apathway therebetween for said tendon, said last mentioned pathway beingaligned with the tangent line to said structure wall.
 3. An apparatus inaccordance with claim 2 in which endless bands move about endless pathsand in which said restraining elements are carried by said endless bandsto engage said tendon and move along said straight line path therewithwhile the tendon is being tensioned and in which said restrainingelements are carried into and from engagement with the tensioned tendonby said endless bands.
 4. An apparatus in accordance with claim 1 inwhich said tendon tensioning means includes means for engaging saidtendon and for displacing with elongation of said tendon duringstressing thereof to prevent slipping of said tendon engaging meansalong the surface of said tendon.
 5. An apparatus in accordance withclaim 1 in which means are provided on said carriage for displacing saidtendon tensioning means transversely to the path of carriage movement toposition adjacent tendons side by side while said carriage remains in agiven path of carriage movement.
 6. An apparatus in accordance withclaim 5 in which means on said carriage engage said structural wall atspaced locations and hold said carriage against twisting on saidstructural wall after a transverse shifting of said tensioning means andtendon.
 7. An apparatus in accordance with claim 1 in which said meansfor moving said caRriage includes caterpillar tracks for engaging saidstructural wall and for providing a propelling force to move saidcarriage about said wall.
 8. An apparatus movable about a wall of astructure for tensioning a tendon and for applying the tensioned tendonto said wall, said apparatus comprising a carriage movable about thewall of the structure, said carriage having a supply unit and a tendontensioning unit, caterpillar track means on at least one of said unitshaving tractive engagement with the structural wall, means pivotallyconnecting said units for movement together as a tandem about thestructural wall, a supply reel of tendon carried on said supply unit forproviding tendon for tensioning, and means on said tendon tensioningunit for tensioning the tendon and paying out the tensioned tendon alonga line substantially tangential to said structural wall.
 9. An apparatusin accordance with claim 8 in which means on said carriage engage saidwall at spaced points and hold said carriage against twisting on thestructural wall.
 10. An apparatus in accordance with claim 7 in whichmeans are provided to shift said tendon tensioning means to permit aplurality of tendons to be applied at longitudinally spaced positionsalong said structural wall without shifting said carriage along saidwall.
 11. Apparatus for tensioning a tendon and for banding a wall of astructure with a tensioned tendon, said apparatus comprising a carriagemovable in a forward or reverse direction about the wall of thestructure, tendon tensioning means on said carriage for tensioning saidtendon while said tendon is moving along a substantially straight linepath which is along a tangent to the structural wall, said tendontensioning means including a series of restraining elements for engagingand restraining said tendon and for traveling therewith while tensioningthe same along an extended linear portion of said tendon, means mountingsaid tendon tensioning means on said carriage for alignment with atangent line extending from said tendon tensioning means to saidstructure, said restraining elements being movable in one direction whensaid carriage is paying out said tendon and being movable in an oppositedirection when said carriage moves in the reverse direction to take upsaid tendon and means for moving said carriage in the forward directionto pay out the tensioned tendon and in the reverse direction to take upthe tensioned tendon while holding said tendon under a controlled,predetermined tension.
 12. Apparatus in accordance with claim 11 inwhich said means for moving said carriage includes a motor drive andcaterpillar tracks for engaging the structural wall and in which saidtendon tensioning means includes a pair of endless carriers for carryingsaid restraining elements into engagement with said tendon to traveltherewith and from said tendon for return to positions to re-engage saidtendon.
 13. A prestressing apparatus as defined in claim 12 wherein saidrestraining elements are slidably mounted and have tongue and yokeportions, said yoke portion including a pair of flanges, and the tongueportion of each element adapted to be slidably positioned within theflanges of an adjacent element.
 14. A prestressing apparatus as definedin claim 12 wherein said restraining elements include a resilient memberwhereby the sliding movement of the restraining elements in passingthrough the tensioning zone is substantially equal to the elongation ofthe tendon in passing through said zone.
 15. A prestressing apparatusfor circumferentially tensioning a tendon about a continuous structureby elongating the tendon a desired extent, said apparatus comprising asupport carriage movable about said structure, a tendon tensioningmechanism carried by said carriage means mounting said tendon tensioningmechanism on said carriage for articulation about an axis to align saidtendon tensioning mechanism with a line tangent to said structure, andmeans for shifting the mechanism aLong a path parallel to said axis toadjust said line of tangency relative to said carriage and to saidstructure.
 16. A prestressing apparatus as defined in claim 15, whereinthe tensioning mechanism includes a pair of opposed loops each comprisedof flexible links engaging a pair of spaced, rotatable sprockets fixedlypositioned to maintain said loops taut, means for rotating a sprocket ineach pair in timed relation whereby said loops define a pathway for thetendon and create a zone for progressively increasing the tension insaid tendon as it passes through said zone.
 17. A prestressing apparatusas defined in claim 16 wherein said opposed loops include a plurality oftendon restraining elements for engaging the tendon throughout itstravel along the pathway defined by the opposed loops.
 18. Aprestressing apparatus as defined in claim 17 wherein said tendonrestraining elements have a surface thereon for engaging the tendon, thehardness of said surface being less than that of the tendon.
 19. Aprestressing apparatus as defined in claim 16 and further including aback-up plate positioned within each loop and extending substantiallythe length of the pathway formed by the loops, at least one of saidplates adapted to be moved whereby the pressure exerted by therestraining elements on the loops against the tendon may be varied. 20.A prestressing apparatus as defined in claim 17 wherein said restrainingelements are supported by a plurality of rollers whereby the distancebetween the holding elements of the opposed loops is determined by thedistance between the opposed rollers.
 21. A prestressing apparatus asdefined in claim 15, in which a movable carrier for the tendontensioning mechanism is provided on said carriage and said tendontensioning mechanism is supported thereon in a cantilevered condition,and in which a counterweight arrangement comprising a flexible connectorof a fixed length is attached to one side of the cantilevered portion ofthe tensioning mechanism and passed over a roller fixed on the supportcarriage and attached to the carrier on the side thereof opposite theattached side of the cantilevered portion whereby said tensioningmechanism is supported by said connector as the carrier is movedrelative to the carriage.
 22. A tendon mechanism for prestressing atendon without substantially bending the prestressed tendon beforebanding the tendon on a structure having a longitudinally extending axiscomprising, a frame means, a support means, a pair of opposed, endlessbands mounted on said support means for movement about endless paths,and a plurality of tendon restraining elements mounted on said endlessbands for travel therewith and for forming a straight line pathway forthe tendon to travel along, said restraining elements engaging thetendon during its travel along said straight line pathway, said tendonrestraining elements exerting forces on said tendon to prestress thesame while traveling along said straight line pathway and meanspivotally mounting said support means on said frame means for pivotingthereon about an axis parallel to said longitudinal axis of thestructure and relative to said structure being banded to align saidtendon being tensioned with a tangent line to the structure.
 23. Atendon tensioning mechanism in accordance with claim 22 in which saidsupport means is slidably mounted on said frame means for movement inthe direction of tendon travel and in which load sensing means areoperated upon by said support means moving in the direction of tendontravel to provide an indication of the prestressing force being appliedto the tendon.
 24. A tendon tensioning mechanism in accordance withclaim 22 in which means are provided to apply force to said tendonrestraining elements in a direction normal to the tendon travel to holdthe tendon against substantial slippage relative to said restrainingelements.
 25. A tendon tensioning mechanism in accordance with claim 24in which means are proVided to allow said restraining elements to moverelative to said endless bands during elongation of the tendon toprevent substantial surface slippage between the tendon and therestraining elements.
 26. A tendon tensioning mechanism in accordancewith claim 22 in which means are provided to allow said restrainingelements to move relative to said endless bands for a predetermineddisplacement during tendon elongation and in which surfaces are providedon said restraining elements for partially yielding to accommodatefurther elongation of said tendon to its fully stressed length.
 27. Amethod of tensioning a tendon and banding a circumferentially extendingwall of a structure with the tensioned tendon by a tendon tensioningmechanism carried on a movable carriage; said method comprising thesteps of: moving the carriage circumferentially about the wall,tensioning an extended linear portion of said tendon while said tendonis moving along a straight line path through said tendon tensioningmechanism without producing a substantial bending of the tendon andwithout drawing the tendon to produce the tension therein, aligning thetensioned linear portion of the tendon with a straight line tangent pathfrom the tendon tensioning mechanism to the circumferential wall of thestructure, paying out said tensioned tendon along said straight linetangential path to said structure, and banding said circumferential wallwith said tensioned tendon.